The Real-time Transport Protocol (RTP) and its related standards define a retransmission packet format and a way to give feedback via Negative ACKnowledgement (NACK) packets that data has been lost. The following standards RTP (RFC3550), RTP Retransmission (RFC4588), RTCP Feedback (RFC4585), and RTCP with SSM Sessions (draft-ietf-avt-rtcpssm-11.txt) are all incorporated by reference and describe unicast feedback and retransmission for unicast sessions, and unicast feedback with multicast retransmission for multicast sessions.
However, the RTP protocol suite has limitations when used with certain types of Internet Protocol (IP) media transmissions, such as transmitting streaming video to different endpoints. For example, neither RTP, nor any other common media transmission protocol, can perform unicast repair of multicast media streams or quickly and efficiently switch among different multicast media streams without loss of data.